As the position of the Earth's
axis changes, it will point in slightly different directions and therefore at
different stars. At the moment, it's northern end is ponting to within a degree
of Polaris in Ursa Minor. In about 12,000 years from now, it will be pointing
at a point some 12 degrees from Vega, the main star in the constellation of
Lyra. The star Thuban (in Draco) was the star closest to the pole about 4500
years ago.

The precession of the Earth's
axis is caused mainly by the gravitational pull of the Sun and Moon. There is
a smaller component caused by the gravitational pull of the planets. The overall
effect of precession is to move the axis of the Earth by just under 1 degree
every 70 years or so. A star atlas needs to take account of this and needs regular
updating if the positions of the stars and other objects are to be correct.
A good star atlas such as Nortons will give the star positions for a particular
time (referred to as the epoch). The current epoch for star catalogues is 2000
AD. Small corrections (given in tables supplied within these publications) need
to be applied for dates other than the epoch date for precise work. For general
work, they are accurate enough without alteration.